The Sand Box

A Magical Puzzle Key for opening a door. Innocuous, somewhat simple, but fun to solve, if you like puzzles.

This is an item that is basically a magical lock to a nearby secret and impenetrable door. It’s a way to sneak an interesting puzzle into the game, basically.

In an otherwise empty room is a long, anvil-like piece of stone. The top is flat, and inscribed on the left half of the top are a sequence of figures. To the right of those figures is a small bowl filled with sand. The sand is level and unmarked.

Nothing bad happens if the sand is disturbed, and the sand can be returned to a state of “clean slate”, simply by flattening the sand. The “anvil” is stuck to the ground, and is immovable.

There is a nearby secret door that is found but can’t seem to be opened. The only thing in the room is this anvil-like thing with the figures on it.

The sequence looks like this(I hope the link works, cut and paste if you have to):
http://www.goslin.info/various/sequence-puzzle.jpg
(Feel free to download and use that image if you want.)

Once the appropriate next symbol is scribed in the sand, the secret door pops open. Can you figure out the next symbol?

ANSWER: Draw two 8’s next to each other, like this 88. The symbols in the diagram are mirror imaged numbers from 1 through 7.

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Well... to me this was kind of obvious as soon as I saw the picture, even if I never saw it before. Then again, I showed this to three other people and only one of them got it, and had to think about it for a while. So... I guess it's actually a good puzzle, and I'm just wired this way?

Edit: Welp, I just tried to see how it turned out with my group, and they told me they saw this in The Simpsons. Guess it's probably not gonna be an easy puzzle to add without everyone knowing it beforehand.

Thank you tobydjones for the link! The image is pretty important.
Post delivers on its promise: a simple, straightforward puzzle you can use pretty much anywhere. Use of sand as an input device is a nice addition.